Enable Dual GPU Cuda on MacOs Seirra
Are your GPUs in SLI or similar pairing technology, or somehow presented at OS level as a single GPU?

If so that will negatively affect performance in Blender, because those systems are generally inefficient and will not fully utilize the power of both GPUs entirely.

Blender however if presented with two discrete graphics cards for rendering in Cycles can make full use of both at full speed without performance compromises, virtually doubling the speed and halving render time.

Having only one bucket while rendering is generally a sign that only one GPU is being detected by Cycles.

If possible tweak your settings in such way that Blender can see both your graphics cards as independent hardware.